1. Technical Field
This invention relates to electrical transformers and, more particularly, to a broad band transmission line transformer with inherent DC isolation.
2. Background Art
Ideal transmission line transformers with integer turns ratios but without DC isolation are well known. These transmission line transformers are considered ideal because when the even mode impedance is suppressed and the even mode characteristic impedance is very large, the response is independent of frequency while the transmission lines remain in the TEM mode.
Inherent DC isolation in such a transformer is especially desirable in high power amplifier designs, since high voltage, high current, high Q coupling capacitors can be eliminated. DC isolation transmission line transformers are available, but these are substantially frequency dependent. In NASA Tech Briefs, "Coaxial Cable on Toroid Yields Wide-band Transformer", Electronic Design, pp. 108-109, Jun. 21, 1967, such a transformer is described, but its frequency response falls above ten Mhz. A similar transformer is shown in H.D. Granberg, "Broadband Transformers and Power Combining Techniques for RF", Motorola Application Note AN-749, 1975, but it has considerable performance degradation for frequencies less than 175 Mhz. Other DC isolation transmission line transformers are shown in H.O. Granberg, "New MOSFETs Simplify High Power RF Amplifier Design", RF Design, PP 43-52, Oct. 1986 and H.O. Granberg, "Building Push-Pull Multioctave, VHF Power Amplifiers", Microwaves and RF, pp. 77-86, Nov. 1987.
The present invention was developed from an attempt to modify the prior art transmission line transformer 10 of FIG. 1 to render it frequency insensitive over a wide band frequency range. In this prior art transformer 10, a plurality N of substantially identical transmission line elements LE-1 through LE-N, generally referred to as LE-N, are provided in the form of coaxial cables. All of the coaxial cables have the same characteristic impedance Z, and all have the same electrical length. The primary 12 of each transmission line element LE-N is preferably defined by a center conductor of a coaxial cable segment, and the secondary 14 is preferably defined by an outer conductor. The coaxial cable segments are turned around one or more ferrite cores 19.
The primaries 12 of all of the transmission line elements LE-N are connected in series between a pair of input terminals P1 and P2 by means of suitable electrical connections 16. The input terminal P1 is connected between a source VS of AC signals through a series source impedance RS, and the input terminal P2 is connectable to ground at the other side of source VS. The secondaries 14, on the other hand, are connected in parallel with each other and with a pair of output terminals P3 and P4. Output terminals P3 and P4 are shown connected with a load such as a coaxial feed line to a transmitting antenna having an impedance RL.
The theoretical frequency response of the prior art transformer of FIG. 1 is shown in FIG. 2 in solid line plots for N=1 through N=7 for different, nonzero, equal values of characteristic impedances Z of LE-N. RL=1 is assumed for simplicity. The abscissa variable is the length of the transmission line winding in percentage of wavelengths and the ordinate variable is the transmission gain in dB. Since the lines are assumed lossless, the transmission gain and loss is also equal to the mismatch gain and loss.
Each transmission line element LE-N comprises one turn. Accordingly, there are N equivalent turns on the primary side connected to load RL. The secondary side consists of N turns, but since they are connected parallel, the N turns are equivalent to a single turn. Thus, a turns ratio of N:1 is realized corresponding to an impedance ratio of N.sup.2 =RS/RL.
The solid line plots in FIG. 2 are for values of Z=ZO from the equation (1) below recommended in the first reference of Granberg discussed above. ##EQU1## As shown in FIG. 2, there is substantial frequency sensitivity for all recommended values of ZO between one and 21 for values of N between one and seven.